Sunday, October 3, 2010

Nitrogen Cycle for Dummies

     I noticed that the nitrogen cycle is not well understood and respected by new aquarists and I thought to explain it in a more accessible way. Most people who have this hobby are abandoning it because they become frustrated when the fish begin to die. Usually they go and buy new fish. Most of these disasters can be avoided by understanding the nitrogen cycle and closely follow water parameters.
     Fish release ammonia through excrements. Ammonia is released also when excessive food and dead fish are left to decay in the aquarium. That's why is vey important not to feed your fish too much and maintain a high level cleaning in the aquarium. Ammonia is toxic for the fish, when its levels increase greatly is because bacteria called Nitrosomonas have not yet developed. They are the ones that break down ammonia into nitrite (NO2), substances that are less toxic than ammonia.
     Nitrosomonas bacteria will adhere to any surface, such as substrate, inside the filter, the filter sponge, etc.. In the first two weeks will notice a decrease of ammonia and nitrite levels will begin to increase. After that nitrite will be decomposed by nitrobacteria to nitrate. Nitrobacteria will adhere on the same type of surfaces like Nitrosomonas. Nitrites can be dangerous for fish if allowed to exceed a certain threshold of safety, usually > 0.3 mg / l. When it is revealed that the level of nitrites is reduced, you can expect to increase the level of nitrates.
     In a new aquarium, you can see a slight increase of ammonia and of course of nitrites, when you add a significant number of fish. An aquarium "mature" should not show detectable levels of ammonia and nitrites. If these substances are present in the tank, you should determine the causes and solve the problem as quickly as possible.
     Nitrates (NO3) are not as toxic as nitrites. Most fish tolerate high levels of nitrates if the growth is slow. Fish that are in such an environment will not live very long and will not reach their full potential. Another problem with high levels of nitrates occur when you add new fish in the tank. Usually, pollutants from the water are too numerous for new fish to adapt, and that's why they get sick or die. High level of nitrates is responsible for reducing resistance to disease. Plants use nitrates from the water as food and if there are excessive amounts of nitrates, plants will stor them inside. This will lead to poisoning plants with nitrates. It will be noticed that their leaves no longer grow and eventually the plant will die. As a general rule, a level of nitrates between 25-50 ppm will not harm the fish or slow their growth.
     In general, for most people, the easiest way to reduce nitrates is to change the water. Nitrosomonas bacteria and nitrobacteria are aerobic and nitrobacteriile, which means they need oxygen to survive. Nitrosomonas bacteria need oxygen to survive. Their numbers may be affected by lack of oxygen, such as substrate  without oxygen or areas that do not receive enough oxygen because of poor water flow. When you clean the tank, be careful not to clean it too well. Do not clean the filter and substrate in the same time,you could be risking to lose significant portion of the beneficial bacteria. Chlor and chloramines will kill them, as well as hot water, so be careful when you rinse the filters.
     Both Nitrosomonas and nitrobacteria need micro-nutrients that are not found in reverse osmosis water, distilled water or other purified water. Some antibiotics will kill the beneficial bacteria. When water from the tank is treated with antibiotics, check if there is an increase in pollution. Exceeding the safety threshold depends on tolerance and fish species that you have in the tank. Try to start with more resistant species and keep the ammonia and nitrite to the lowest level indicating on the scale that comes with the test kit. In a new aquarium, you should not remove all the ammonia and nitrite because bacteria need food to survive. When you see rapid growth of these levels you should make partial changes of water.
     In general, the nitrogen cycle lasts between 4 and 8 weeks, sometimes longer. At one week after setting the tank, necessary time for plants to make roots and water to remove dissolved gases, first fish can be put in the tank, preferably resistant species, Corry is a good example to start the nitrogen cycle. These fish will be affected and will die or will live less than average. If not enough fish in the tank, cycle will not start, and if too many fish, they will die because of excessive amount of ammonia. The number of fish that is recommended to be placed in the tank depends on the size of the fish, filter, etc..
     After 3 days of adding fish ammonia is tested and continue so until ammonia begins to decrease, reaching zero. If you see that the fish are breathing quickly, swimming disorderly and stay at the surface, take measures to lower ammonia through water changes. Nitrites will be tested a week after introducing the fish. Water exchanges will be made until nitrites will reach zero. When nitrites are reduced, nitrates will grow and will be kept within limits by water changes. After about two weeks nitrates will be tested  and maintained within normal limits through water changes. At one point, nitrates will drop, this being a sign that nitrosomonas bacteria were developed. When they reach to normal values, fish can be introduced , but in limited number. The pH also increases during this process, then returns to normal values. Weekly, few fish can be inserted until it reaches the desired number of fish in the aquarium. Frequent water exchanges in smaller amounts are recommended, about 25-30% of the tank volume, so the pH shock doesn't occur.
     What you shouldn't do:
  • DO NOT add other fish, wait for the nitrogen cycle to end.
  • DO NOT change the filter media, they are support for the beneficial bacteria.
  • DO NOT disturb the colonies of bacteria until they are stabilized.
  • DO NOT feed your fish too much.
  • DO NOT attempt to modify the pH because nitrifying bacteria can be affected by this; in the nitrogen cycle is normal that pH to be increased, it will return to normal at the end of the cycle.


  1. the nitrogen cycle doesnt end. it equals out and there may be some spikes when you test the water.

    (not gonna type much more...i have a headache...)

    when the test levels spike,dont panic. you could cause more harm than help to your fish...

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  5. You can cut down the time required for fishless cycling by using heavy bacteria seeding. There are proven working commercial available live bacteria products such as Tetra Safestart. Such products can cut down the cycling time to as short as a few days. Check out the following article on the quickest way of fishless cycling.
    Aquarium Nitrogen Cycle and Fishless Cycling

  6. this isn't for dummies its for smart people.